As the density level in storage devices such as magnetic disk devices and optical disk devices has been growing, a variety of techniques have been used in recent years to acquire data correctly from the signals read from the storage devices. Examples of the conventional techniques include error detection, correction processing, and retry processing by which the reproduction operation is retried when a reproduction error is detected. For example, in the retry processing performed in an optical disk device, signal quality of reproduced data is improved by changing various retry parameters such as focus offset or signal amplitude and reproducing again the error location.
Further, a method for reproducing a plurality of times the same region and calculating an arithmetic mean of the same waveform by using a memory circuit (see, for example, Patent Literature 1 and Patent Literature 2) and a majority decision method of comparing a plurality of data that have been read out and using the data with the highest occurrence frequency (see, for example, Patent Literature 3 and Patent Literature 4) have been suggested as techniques for increasing signal quality.
A LDPC (Low-Density-Parity-Check) code has recently been suggested as a powerful error correction code in satellite broadcasting standards and next-generation communication standards. In the LDPC code, by contrast with the conventional algebraic error correction codes, stochastic inference calculations are used for decoding processing, thereby making it possible to demonstrate the performance close to the Shannon limit. By using such an error correction technique, it is possible to perform error correction even with respect to reproduced signals with an extremely poor error rate. Therefore, the aforementioned error correction technique has started finding applications in a variety of fields with the object of increasing the recording density and communication capacity.
However, in a reproduction system such as uses the aforementioned error correction technique, the quality of the reproduced signals is poor. Therefore, it is very difficult to sample a plurality of reproduced signals at the same timing in each cycle. With the conventional configurations disclosed in Patent Literature 1 and Patent Literature 2, it is necessary to average a plurality of reproduced signals and therefore a plurality of reproductions should be performed accurately at the same timing. However, when the quality of the reproduced signals is poor, the signals are very difficult to reproduce at the same timing, and in extreme cases, a bit slip occurs in a synchronization circuit (PLL (Phase Locked Loop) circuit). The resultant problem is that a synchronization shift occurs in waveform sampling, and the reproduction performance is greatly degraded.
The occurrence of a bit slip will be explained below in a simple manner with reference to FIGS. 17 and 18. FIG. 17 shows an example of sampling data during normal operation. FIG. 18 shows an example of sampling data in the case a synchronization shift has occurred. In the optical disk recording and reproduction device described in Patent Literature 2, the digital signal obtained by converting the reproduced signal read from an optical disk into a digital value with an A/D converter is saved in a memory, and the averaging is performed by using two reproduced waveforms. As shown in FIG. 17 where the reproduced signals are acquired at an adequate sampling timing during normal operation without the occurrence of a synchronization shift, the S/N ratio can be improved by 3 dB at a maximum.
By contrast, where the synchronization shift (timing shift) occurs, as shown in FIG. 18, the averaging cannot be correctly performed by using the reproduced waveform of the first cycle and the reproduced waveform of the second cycle, and multiple reproductions degrade the error rate with respect to that obtained with a single reproduction. Furthermore, the waveform of the reproduced signal averaged when the synchronization shift is induced is entirely different from the original reproduction level. Therefore, in the signal processing system, the usual synchronization processing of reproduced signals by the synchronization pattern or the like cannot be performed and signals cannot be sent to the error correcting device at a correct timing.
In the conventional devices in which a plurality of reproductions is performed, the abovementioned problem mainly occurs due to the averaging of the reproduced signals performed before the synchronization. In a system such that uses the LDPC code, the error rate of the reproduced signals prior to error correction is high and the abovementioned problem becomes more serious.